In a process of that kind which is basically known a polymer in the form of a polymer melt or in the form of a solution is introduced into an electrical field and spun to form fibers by virtue of the action of an electrical field. In that procedure an electrode usually forms a receiving device for the spun fibers while the counterpart electrode is frequently in the form of a spray nozzle. The last-mentioned electrode can however also be in the form of a heatable conveyor belt which can be charged up at a given potential, for converting solid polymers into a molten material and for spinning fibres from that molten material.
As an example of a process for the production of a corresponding fiber product from a polymer melt, reference may be made for example to U.S. Pat. No. 4,230,650.
A particularly preferred area of use of non-woven fabric materials is use as filter media. In the production of such filter media the nano- and/or microfibers which are produced using an electrostatic spinning process are generally not deposited in isolation but directly as a non-woven fabric or fleece.
A process of that kind is known for example from U.S. Pat. No. 4,144,553. In addition there are a large number of proposals for improving electrostatic spinning processes, which concern an improvement in the electrodes and the electrode geometry as well as the compositions of the polymer melts and/or polymer solutions used. It was possible in that way in particular to influence fiber thickness, fiber thickness distribution and the length of the nano- and/or microfibers produced, as well as the structure of the non-woven fabric produced.
The usual electrostatic spinning processes, for deposit of the fibers, generally involve using a counterpart electrode made of metal, over which a textile carrier material is mostly passed, with the nano- and/or microfibers being deposited on the carrier material to constitute a non-woven fabric. The charge is partially removed from the deposited fibers. That is certainly desirable in relation to some uses, for example as filters. On the other hand the residual charge of the fibers which have already been deposited prevents the further deposit of fibers which are charged in the same direction. That means that the total charge of the non-woven fabric is limited by the electrical voltage at the counterpart electrode and at best the procedure involves continuous charge removal in the operation of depositing further fibers.
That has the disadvantage of limiting the filter action which can be achieved as, besides the screening effect of the fine fibers, that filter action is largely based on electrostatic separation of particles at the fibers. The electrostatic charge is fixed in a certain manner in the spun nano- and/or microfibers while the counter-charge can flow away to the charged fibers by way of the non-woven carrier material and at the fibers can neutralise the charge thereof.
Those limitations as a matter of principle can be only partly circumvented by the subsequent separation of oppositely charged fibers, as described in DE 20 320 72 A. The problem still persists that separation takes place only as long as the attracting voltage of the counterpart electrode overcomes or exceeds the repelling voltage of the deposited fibers.
Therefore the object of the present invention is to provide an improved electrostatic spinning process for the production of non-woven fabric materials, in which nano- and/or microfibers are produced from a polymer melt and are deposited to form a non-woven fabric. In particular the object of the invention is to at least partially overcome the disadvantages known from the state of the art, and to provide a process with which it is possible to obtain non-woven fabric materials with properties which are advantageous in particular for filter purposes.